There is a demand in technology for materials which are resistant to heat shock and suitable for operating temperatures above 900.degree. C. and at the same time have a high level of mechanical properties enabling them to be used for engineering constructions such as those required in fusion metallurgy, e.g. throughflow governors, in the construction of apparatus, e.g. heating gas ventillators, in the construction of motor vehicles, e.g. thermal insulators of the waste gas duct (portliners), and in chemical technology, e.g. filters or catalyst carriers.
Although ceramics based on pure aluminium titanate or tialite have interesting properties, such as a low coefficient of thermal expansion (WAK) and a low modulus of elasticity, these ceramics are limited in their technological application owing to their very low strength and their tendency to decompose at temperatures in the region of about 900.degree. C. to about 1300.degree. C. In this temperature interval, tialite decomposes into the starting oxides, Al.sub.2 O.sub.3 and TiO.sub.2, and this decomposition is accompanied by a sharp rise in the coefficient of thermal expansion.
For the preparation of sinterable aluminium titanate powder, it is proposed in U.S. Pat. No. 3,825,653 to co-precipitate halogens, nitrates or alkoxy compounds of aluminium and titanium as hydroxides and to use the co-precipitate after drying and calcining for the sintering of aluminium titanate products. The products prepared by this method should have a thermal expansion in the longitudinal direction of less than 1.times.10.sup.-6 /.degree.C. in the temperature range of from 25.degree. to 1000.degree. C. Figures given for the ratio of Al.sub.2 O.sub.3 to TiO.sub.2 are 1:1 and from 1:3 to 3:1. The addition of compounds of other elements is not mentioned. The pressure resistance is only 34.5 mPa. The powder described in DE-C 2 345 778 is used according to U.S. Pat. No. 3,825,653 for the production of melting crucibles for uranium and uranium alloys. An equimolar ratio of Al.sub.2 O.sub.3 to TiO.sub.2 is regarded as suitable for the method of production by the hot press process. Owing to their low green density due to their very small particle size, the co-precipitated powders have a shrinkage of 50% after calcination.
According to U.S. Pat. No. 3,607,343, Al.sub.2 O.sub.3 particles have been covered with a quantity of from 1 to 50 volumes % of TiO.sub.2 by the flame spraying process (plasma spraying). The Al.sub.2 O.sub.3 particles were put into the reaction with the addition of a suitable binder, e.g. one based on a phenol resin.
P. A. Brugger and A. Mocellin in Journal of Materials Science 21 (1986) 4431-4435 treat Al.sub.2 O.sub.3 or TiO.sub.2 particles with the other element by hydrolytic decomposition of the corresponding organometallic alcoholate compound. According to J. Am. Ceram. Soc. 69 (2), C22-C24 (1986), H. Okamura, E. A. Barringer and H. K. Bowen also apply titanium oxide hydrate to Al.sub.2 O.sub.3 particles by stepwise hydrolysis of a titanium alcoholate solution. The costs of metal alcoholates and the complicated method of carrying out the reactions stand against the industrial utilization of this process.
In Advanced Ceramic Materials 2 (4) 798-803 (1987), S. Kratohvil and E. Matijevic describe the covering of TiO.sub.2 particles with an aluminium oxide component of Al.sub.2 (SO.sub.4).sub.3 solution with the aid of urea. Owing to the low concentration of the Al.sub.2 (SO.sub.4).sub.3 solutions used and the large quantity of urea, this method is of no interest for industrial application.
In Zhurnal Prikladnoi Khimii, Volume 58, No. 10, pages 2207-2211, October 1985, L. I. Bekkerman investigated the precipitation of small quantities of aluminium hydroxide (0.58-3.03% by weight, based on TiO.sub.2) on TiO.sub.2 pigments.
In Report of the Research Laboratory of Engineering Materials, Tokyo Institute of Technology 9, 1984, 75-86, E. Kato, K. Daimon, J. Takahashu, R. Kato and K. Hamano describe the preparation of an Al.sub.2 TiO.sub.5 raw material powder by the treatment of a solution of TiOSO.sub.4 and Al.sub.2 (SO.sub.4).sub.3 at elevated temperatures in a vacuum evaporator. One disadvantage of this process is the considerable quantity of SO.sub.2 formed as gaseous product of decomposition in the reaction and another is the large amount of shrinkage of the powder when sintered.
Numerous proposals for the preparation of ceramics based on aluminium titanate have been made, in which the starting components, Al.sub.2 O.sub.3,TiO.sub.3 and an exactly predetermined quantity of one or more suitable additives are introduced separately before the ceramic working up. The relevant state of the art is described e.g. in DE-C 3 706 209.
It is an object of this invention to provide suitable raw material powders which do not have the disadvantages described above. It has now been found that these requirements are fulfilled by raw material powders based on aluminium titanate containing, in addition to the tialite components, TiO.sub.2 and Al.sub.2 O.sub.3, other suitable oxides of elements distributed very homogeneously.